Magnetic Field and Pressure Dependence of Small Angle Neutron Scattering in MnSi

We report small angle neutron scattering of spontaneous and magnetic field aligned components of the helical spin polarization in MnSi for temperatures $T$ down to 0.35 K, at pressures $p$ up to 21 kbar, and magnetic field $B$ up to 0.7 T. The parameter range of our study spans the first order transition between helical order and partial magnetic order at $p_c=14.6\mathrm{kbar}$, which coincides with the onset of an extended regime of non-Fermi liquid resistivity. Our study suggests that MnSi above $p_c$ is not dominated by the remnants of the first order transition at $p_c$, but that an unidentified mechanism favors stabilization of a new ground state other than helical order.

Figure 1

(a) Sketch of the experimental setup. See text for further details. (b) Rocking scans of FC intensity at $p=0$ and high $p$. (c), (d) Typical SANS pattern (polarized neutron data), where the intensity around the center is due to the pressure cell. The spot to the left-hand side in panel (c) is due to the helical order as aligned in a magnetic field $B\parallel ⟨110⟩$. Inversion of the neutron spin with respect to $B$, shown in panel (d), leaves behind a residual intensity associated with the finite flipping ratio ($\sim 22$).

Figure 2

Typical magnetic field dependence of the SANS intensity above $p_c$, where $B\parallel ⟨110⟩$. Data were recorded at 1.5 K and 0.35 K in panel (a) and panel (b), respectively. Gray shading shows the BG signal. The magnetic field history following cooling under zero applied field is: (1) ramp to 0.7 T, (2) ramp down, and (3) ramping field up again. Inset: Exponent $\alpha$ of the $T$ dependence of the resistivity as reported in Ref. 4.

Figure 3

Intensity for different $\varphi$ between the field direction and $⟨110⟩$ (cf. Figure 1), where the sample was rotated around the $⟨111⟩$ axis away from $⟨110⟩$. At each point the sample was first ZFC, followed by a field increase to $B>0.6\mathrm{T}$ and field reduction to the value shown in the plot. Open symbols (⊞) show the signal on the opposite side of the beam stop and the gray shading indicates the BG.

Figure 4

$T$ dependence of the spot intensity for ZFC, FC, and 0.4 T at $p=21\mathrm{kbar}$ and $B\parallel ⟨110⟩$ after a field history equivalent to curve “2” in Fig. 2b. Below a few $K$ the intensity increases logarithmically. The inset displays the data as scaled by a constant. The point size is larger than the error bars.

Figure 5

(a) Pressure dependence of the ordered magnetic moment squared $m_s^2$ as reported in Refs. 14, 15. (b) Characteristic field values seen in the resistivity, susceptibility, and SANS. Blue and green shading indicate the regime of the FL and NFL resistivity, respectively. In contrast to $m_s$ no significant pressure dependence is seen for $B_{c1}$ and $B_{c2}$.